Diseases caused by Trypanosoma brucei spp. represent health hazards for many disadvantaged countries. Because of failing vector control and the rise of drug-resistance, the re-appearance of African sleeping sickness necessitates a search for new drug targets with emphasis on parasite-specific processes. Trypanosomal mitochondrial RNA processing pathways are extremely divergent from those of humans and represent a potential source of therapeutic intervention points. The mRNA editing has been extensively studied, but the significance of pre- and post-editing processing events emerged only recently. We discovered the mitochondrial poly (A) polymerase (KPAP1) and terminal uridyltransferase (RET1), and showed that intertwined polyadenylation and uridylation processes are critical for mRNA stability, translation and decay. In these studies, I encountered a diverse family of pentatricopeptide (PPR, 35 amino acids) helical repeat- containing RNA binding proteins that populate polyadenylation and translation complexes. By focusing on mRNA 3' end definition, modification and stability, this proposal introduces PPR proteins as key regulators of mitochondrial gene expression. The experimental approaches involve biochemistry, proteomics, crystallography, genetics and deep RNA sequencing. The proposal aims to: 1) Dissect general and transcript-specific mRNA adenylation/uridylation mechanisms. I hypothesize that the two modes of 3' modification, pre-editing addition of a short A-tail and post-editing A/U-tailing, serve discrete purposes of mRNA stabilization and commitment to translation, respectively. 2) Determine principles of uridylation- based mRNA decay. A hypothesis that RET1 TUTase-catalyzed uridylation accelerates mRNA 3'-5' degradation will be tested. I also posit that most transcripts are stabilized by a specific PPR protein that blocks mRNA uridylation. 3) Elucidate functions of a putative mRNA 3' definition and stabilization factor. I suggest that a PPR factor is deposited onto the pre-mRNA prior to polyadenylation to define the 3' end. We focus on select PPR proteins in trypanosomes, but the long term goal is to illuminate the mechanisms of RNA recognition by repeat-containing proteins, which will be of fundamental importance for RNA processing and mitochondrial biology fields.